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Think about a small block resting on a larger block. If I move the large block forward, the static friction causes the small block to move in the same direction as the displacement, so the work is positive. I can't think of an example of kinetic friction doing positive work, though.

warblersrule, i think that is more inertia than friction... work of friction < = (coefficient of friction) * force of the normal... when there is no force acting on the smaller block, friction does no work...

xylem: force of friction is in the opposite direction of the motion... so if you move a block to the right (+) friction does - work... if you move the block to the left (-) the friction does + work

Friction does negative work. Work=friction*r*costheta
when you sum the forces, friction is a negative force because postitive forces are in direction of motion. theta is the angle between friction and r, your displacement. Since friction is in the direction opposite displacement, theta= 180 and work is thus negative. Therefore, friction always does negative work.

I see what you're saying, but the work is still negative even though the friction is positive. Work=r*friction. Friction and r are always in opposite directions. So if r is in the negative direction, then friction is in the positive direction. So work is still neagive because a negative *postive is still negative.

Another way of looking at it is with the equation Work=magnitude of friction*magnitude of r*cos180. In this equation, you only look at the MAGNITUDES of friction and r, which are always positive. In this case, cos 180 gives you the negative sign and Work done by friction is still negative.

The cos 180 tells you that friction is always opposite the direction of displacement.

What about a moving car? The only force acting on the wheels is the static frictional force between them and the road...it's acting in the same direction as the displacement, and thus is doing positive work.

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